# FRET Calculator The efficiency of fluorescence resonance energy transfer (FRET) from a donor fluorophore to a nearby acceptor depends on a number of parameters, including the quantum yield of the donor, the extinction coefficient of the acceptor, the spectral overlap of donor emission and acceptor absorbance and the dipole orientation. The FPbase [FRET calculator](http://www.fpbase.org/fret) predicts the Förster Distance (i.e. the distance between donor and acceptor that will result in 50% FRET; R0) for all monomeric FP pairs in the database for which the required parameters are available. ![FRET Calculator showing](/files/-LWh20wojwcRwQuG0KqE) Förster distance values are calculated using the method presented in Wu & Brand (1994). Additionally, Förster distance multiplied by the quantum yield of the acceptor is calculated as an indication of dynamic range expected in ratiometric FRET using the sensitized emission method. Results are presented in a sortable tabular format, with a search field for ease of locating specific proteins. The overlap integral (between donor emission and acceptor absorbance) is highlighted in the spectra image for the currently selected FRET pair, and values for refractive index and orientation factor can be changed by the user. {% embed url="" %} --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://help.fpbase.org/tools/fret-calculator.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.